I own a Passion PAK A11 stereo power amplifier which uses four 12AU7 tubes
in the first two stages and four KT88 tubes for the final power stage. The
amp employs a push pull design. The last time I used the amp I heard a pop
then a high pitched whine and when I checked the amp one of the KT88 was
glowing red hot and the circuit board was smoking and almost on fire. The
KT88 cathode resistor and the resistor between the second grid and output
transformer were burned out. The first grid is coupled the first stages with
a coupling capacitor and connected to the biasing circuits. I tried
replacing the burned out resistors and the potentially damaged tube and
slowly powered up the amp with a VARIAC. At first all seemed ok but the bias
voltages were off and the new tube soon began to glow red. I quickly shut
off the power. Clearly the problem was not the damaged resistors. I suspect
any one of the following in descending order:

1.) Coupling capacitor bad.
2.) Bad 12AU7 tube in first stage.
2.) Problem in the biasing circuitry (possible bad transistor).
3.) Output transformer bad.

I'm wondering what some of you all think my problem might be? Also, do you
think I have damaged the new KT88 as it was hot only for a short time?
Thanks in advance for your help.

Mike

Kettneraspis wrote

...I suspect any one of the following in descending order:

1.) Coupling capacitor bad.
2.) Bad 12AU7 tube in first stage.
2.) Problem in the biasing circuitry (possible bad transistor).
3.) Output transformer bad.

1) or ...hey! you've got 2 "2"s!

The second 2)

Possibly 3) but that would be a consequence, not a cause. Hope not!

Ensure the bias supply works. You have a variac so you can take the
power valves out and check the bias at the sockets. Watch you don't
exceed the usual HT by taking the variac all the way up.

If there is a problem, disconnect coupling caps one by one. If
problem disappears you have found a leaky cap. Replace and *repeat
tests* in case one fault has led to another.

Why have you got transistors (sshhh!) in your bias supply? They are
devices for the purpose of propagating problems from one part of a
circuit to another.

Also, do you think I have damaged the new KT88 as it was hot only
for a short time?

You will soon find out! Most are pretty tough, so don't worry about
it yet.

cheers, Ian

Ian Iveson wrote:


Why have you got transistors (sshhh!) in your bias supply? They are
devices for the purpose of propagating problems from one part of a
circuit to another.

Igorant stupid **** comment.

Graham

Pooh Bear wrote

Why have you got transistors (sshhh!) in your bias supply? They
are
devices for the purpose of propagating problems from one part of
a
circuit to another.

Igorant stupid **** comment.

Context, Graham! Relax, a little hyperbole is OK on a Sunday. I
quite enjoyed it, anyway:-)

cheers, Ian

Thanks Ian. Sorry about the "2s". I will take your advice to heart. I have
ordered a new cap and tube and will try a repair in a few days. I also hope
the output transformer is ok as I'm not sure I could find a replacement. I
will let you know how it turns out.

Mike

"Ian Iveson" wrote in message
k...
Kettneraspis wrote

...I suspect any one of the following in descending order:

1.) Coupling capacitor bad.
2.) Bad 12AU7 tube in first stage.
2.) Problem in the biasing circuitry (possible bad transistor).
3.) Output transformer bad.

1) or ...hey! you've got 2 "2"s!

The second 2)

Possibly 3) but that would be a consequence, not a cause. Hope not!

Ensure the bias supply works. You have a variac so you can take the power
valves out and check the bias at the sockets. Watch you don't exceed the
usual HT by taking the variac all the way up.

If there is a problem, disconnect coupling caps one by one. If problem
disappears you have found a leaky cap. Replace and *repeat tests* in case
one fault has led to another.

Why have you got transistors (sshhh!) in your bias supply? They are
devices for the purpose of propagating problems from one part of a circuit
to another.

Also, do you think I have damaged the new KT88 as it was hot only for a
short time?

You will soon find out! Most are pretty tough, so don't worry about it
yet.

cheers, Ian